how to reserve a running amazon ec2 instance?

we have a large ec2 instance running in asia pacific region.we want to reserve that particular instance.In aws management console we see an option to purchase a reserved instance but there seems to be no option to change this instance into a reserved one.Are we missing out on something

Reserved instances are a billing feature, not a technical feature.

You can purchase a reserved instance and the discounted hourly rate will apply to already-running instances, without needing to do anything to them. Just make sure the reserved instances you purchase are:

• in the same availability zone as your already-running instances
• of the same instance type (m1.small, c1.medium, etc) as your already-running instances.

As soon as the reservation cost clears your credit card you will see the discounted hourly rate take effect for your already-running instances.

Amazon's docs could be a lot clearer on this. http://aws.amazon.com/ec2/reserved-instances/#4

The AWS's official docs state that the availability zone must be the same. The region is, for example, us-east-1. The AZ is us-east-1a. So, according to AWS, it has to be specific to the last letter.

However, some clever folks found a potential work-around that uses EC2 Consolidated Billing and combines the (odd?) feature that one's us-east-1a might be another us-east-1c (I guess the idea there is to enforce some homogeneity around the AZs).